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A group-theoretic approach to fast matrix multiplication
We develop a new, group-theoretic approach to bounding the exponent of matrix
multiplication. There are two components to this approach: (1) identifying
groups G that admit a certain type of embedding of matrix multiplication into
the group algebra C[G], and (2) controlling the dimensions of the irreducible
representations of such groups. We present machinery and examples to support
(1), including a proof that certain families of groups of order n^(2 + o(1))
support n-by-n matrix multiplication, a necessary condition for the approach to
yield exponent 2. Although we cannot yet completely achieve both (1) and (2),
we hope that it may be possible, and we suggest potential routes to that result
using the constructions in this paper.Comment: 12 pages, 1 figure, only updates from previous version are page
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Which groups are amenable to proving exponent two for matrix multiplication?
The Cohn-Umans group-theoretic approach to matrix multiplication suggests
embedding matrix multiplication into group algebra multiplication, and bounding
in terms of the representation theory of the host group. This
framework is general enough to capture the best known upper bounds on
and is conjectured to be powerful enough to prove , although
finding a suitable group and constructing such an embedding has remained
elusive. Recently it was shown, by a generalization of the proof of the Cap Set
Conjecture, that abelian groups of bounded exponent cannot prove
in this framework, which ruled out a family of potential constructions in the
literature.
In this paper we study nonabelian groups as potential hosts for an embedding.
We prove two main results:
(1) We show that a large class of nonabelian groups---nilpotent groups of
bounded exponent satisfying a mild additional condition---cannot prove in this framework. We do this by showing that the shrinkage rate of powers
of the augmentation ideal is similar to the shrinkage rate of the number of
functions over that are degree polynomials;
our proof technique can be seen as a generalization of the polynomial method
used to resolve the Cap Set Conjecture.
(2) We show that symmetric groups cannot prove nontrivial bounds on
when the embedding is via three Young subgroups---subgroups of the
form ---which is a
natural strategy that includes all known constructions in .
By developing techniques for negative results in this paper, we hope to
catalyze a fruitful interplay between the search for constructions proving
bounds on and methods for ruling them out.Comment: 23 pages, 1 figur
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